INTERCHANGE OF ORBITAL EXCITATION TYPES OF THE LOWEST ELECTRONIC STATES OF 2 RING N--HETEROCYCLICS BY $SOLVATION^{*}$
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Date
1959
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Ohio State University
Abstract
In the one ring N-heterocyclics, e.g. pyridine and the three diazines, the lowest singlet electronic states are $^{1} W$ $(n\pi^{*}$ type) and are clearly separated from the $(\pi\pi^{*})$ $states.^{1}$ In the two ring N-heterocyclics, the be quite different. In the diazanaphthalenes (quinoxaline and phthalazine, the 1,4- and 2,3-diazanaphthalene respectively), the lowest singlet state is a clearly resolved (n, $\pi^{*}$) $state.^{2}$ However in the case of quinoline and isoquinoline (1- and 2-azanaphthalene respectively), the lowest singlet state appears to be of the ($\pi, \pi^{*}$) type. Solvent dependence of the emission spectra proves this to be not the case. The total emission spectra of quinoline, isoquinoline, and quinoxaline were compared with those of naphthalene, and 2-chloronaphthalene, all studied in both hydroxylic and nonhydroxylic rigid glass solutions at $77^{\circ} K$. The fluorescence/phosphorescence yield ratios of naphthalene and chloronaphthalene were independent of solvent. For quinoline and isoquinoline, the ratio increased greatly in a hydroxylic solvent; actually, quinoline showed only a strong phosphorescence in a non-hydroxylic solvent, but a strong fluorescence and phosphorescence in hydroxylic solvent. For quinoxaline, only phosphorescence was observed in all solvents, as expected for a molecule with clearly separated lowest singlet state of $(n, \pi^{*})$ type. The above results can be interpreted as involving the interchange of the lowest $^{1}W(n,\pi^{*})$ and $^{1}L_{b} (\pi, \pi^{*})$ states in the quinolines upon the formation of hydrogen bonded complexes. Such a possibility has been proposed for chlorophyll by several $authors.^{3,4}$ The qualitative features are confirmed by the vapor spectrum studies of quinoline by Mataga $et al, ^{5}$ who found evidence for a lowest $^{1}W$ state. In all of the cases studied the phosphorescence is shown to be from a $^{s}L_{a}$ state. The mean lifetime of the phosphorescences were measured and found to be in the order naphthalene $>$ quinoline $~$ isoquinoline $>$ quinoxaline, which can be understood by the expected differences of spin-orbital interactions in these molecules.
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$^{*}$This research was carried out under a contract between the Office of Naval Research and the Florida State University. $^{1}$ J. W. Sidman, Chem. Rev., 58, 704 (1958). $^{2}$ R. C. Hirt, F. T. King and J. C. Cavagnol. J. Chem. Phys., 25, 374 (1956). $^{3}$ J. R. Platt, in Radiation Biology , ed. by A. Hollaender, McGraw-Hill Book Company, New York, 1956, p. 71. $^{4}$ R. S. Becker and M. Kasha, in The Luminescence of Biological Systems, ed. by F. H. Johnson, American Association for the Advancement of Science, Washington, D. C., 1955, p. 25. $^{5}$ N. Mataga, Y.Kaifu, and M. Koizumi, Bull, Chem. Soc., Japan, 29, 373 (1956).
Author Institution: Department of Chemistry, Florida State University
Author Institution: Department of Chemistry, Florida State University